Materials may be readily characterized in terms of the physical properties which they manifest. Among these properties are reflectivity of various wavelengths of light, electrical conductivity, thermal conductivity, density, specific heat and so forth. Materials may also be characterized in terms of the arrangement of the component atoms thereof. In crystalline materials, atoms are deterministically arranged in highly ordered, repetitive patterns. Amorphous materials in contradistinction, do not exhibit such a high degree of long range order. The term "amorphous" as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions.
The distinction between the amorphous and the crystalline state is an important one for a wide variety of materials insofar as many of the physical properties manifested by those materials will depend upon whether they are in the crystalline or the amorphous state. For example, in many materials electrical conductivity will vary by orders of magnitude for the crystalline and amorphous forms thereof. Likewise, optical properties such as reflectivity, optical absorption and index of refraction may also exhibit significant changes depending upon the degree of crystallinity or lack thereof.
While some materials are constrained to exist solely in the amorphous or solely in the crystalline form, there are a wide variety of materials which can exist in either state. In those instances where the energy barrier separating the two states is of an appropriate value, the material may be readily switched from the amorphous to the crystalline state and vice versa by the input of energy. In some cases this switching may be carried out reversibly, whereas in other cases the reverse path is strongly disfavored, and the material remains in the state in which it is set.
As mentioned previously, the transition between the amorphous state and the crystalline state can produce a diversity of changes in the readily detectable physical properties of a material. Such changes can form the basis for many useful devices and systems. For example, as disclosed in U.S. Pat. No. 3,530,441 of S. R. Ovshinsky entitled "Method And Apparatus For Storing And Retrieving Information", which patent is assigned to the assignee of the instant invention, and the disclosure of which is incorporated herein by reference; both optically and electrically addressable data storage systems may be based upon changes in detectable characteristics exhibited by a material as it is switched between the amorphous and the crystalline state. Similarly, electrical switches, relays and the like may be fabricated utilizing phase change effects.
In light of the significance of the amorphous to crystalline phase change, it would be desirable to be able to control the energy threshold for the onset of this change so as to better exploit the effects thereof. Heretofore, control of the onset of crystallization of a given material has been accomplished by alloying or doping that material with other elements, or otherwise chemically modifying it so as to change its crystallization threshold. While these approaches have generally allowed for a wide degree of control of physical properties of many systems, such control always involved the addition of extraneous substances into the material having its crystallization temperature modified. In some cases the addition of extraneous material could be tolerated, however in other cases such additions changed other properties of the system. Obviously, it would be desirable to have a method for controlling the onset of crystallization temperature in a material, said control being one which does not necessitate the introduction of any extraneous materials into the material.
Control of the onset of crystallization can have significant use in optical and electronic data storage systems such as those discussed previously. Furthermore, control of the onset of crystallization also has use in the more general field of materials research. Also, by controlling the onset of crystallization, the amorphous phase can be "locked in" for a significantly greater temperature range, thereby making it possible to better utilize the unique properties of amorphous materials such as catalytic properties, magnetic properties, optical properties, corrosion resistance and so forth, for electronic devices, surface coatings and the like.
According to the principles of the instant invention, which principles will be described in greater detail hereinbelow, a crystallizable amorphous material may have its crystallization temperature significantly increased without the addition of extraneous material thereto. The instant invention also provides for the fabrication of a multilayered structure in which at least one layer thereof has its crystallization temperature significantly increased. Such structures have great utility as optically addressable data storage devices.
These and other aspects, advantages and utilities of the instant invention will be further described and detailed in the brief description of the invention, the drawings and the detailed description which follow.